In recent years, attention is paid to researches which essentially use a nature of quantum mechanics to realize unprecedented functions of information communication and information processing. For instance, there are pointed-out possibilities that quantum cryptographic communication may realize perfect confidentiality from an eavesdropper based on the uncertainty principle, and quantum computation may perform in a few hours a calculation which requires an astronomical time with an existing computer. An “entangled photon pair source” is an indispensable technology component for realization of those applications. For instance, while distance limitation in quantum cryptography is 100 km or so, there is a proposal of a repeater containing the “entangled photon pair source” as a method for breaking through the limitation. On the other hand, it is theoretically indicated that the quantum computation using photons can be performed by combining the “entangled photon pair source” with existing optical devices and photon detector. Parametric downconversion has been mainly used as the entangled photon pair source so far. As for this method, however, photon pairs are extensively generated in a very wide range. Therefore, only a part of the generated ones can be used, and coupling to a fiber is also very difficult.
As for the entangled photon pair source, the two methods disclosed by P. Kwiat et al. in 1995 (reference #1 and in 1999 (reference #2) have been used so far. However, the photon pairs generated from these photon pair sources are extensively radiated in a wide solid angle so that their yield is low. While experimental researches in quantum information are becoming popular recently, the researches in generation of the entangled photon pair have hardly been made so far.
The reference #1 is “P. G. Kwiat, K. Mattle, H. Weinfurter, A. Zeilinger, A. V. Sergienko, and Y. Shih, Phys. Rev. Lett. 75, 4337 (1995).” The reference #2 is “P. G. Kwiat, E. Waks, A. G. White, I. Appelbaum, and P. H. Eberhard, Phys. Rev. A 60, R773 (1999).”
It is an object of the present invention to realize an entangled photon pair generating apparatus capable of efficiently generating a photon pair in an entangled state by converging them in beam shape.